virgl: force linear texturing support

When I made sure that half-float texture-filtering was required for ES3, I didn't realize that virgl doesn't report support for this correctly. This regressed the GLES version available on top of several drivers, including i965 from 3.2 to 2.0.

This is going to need protocol changes to fix properly, so let's just restore the previous behavior by enabling floating-point filtering unconditionally for now.